Transmit power control for mobile unit

ABSTRACT

In a wireless local area network which includes access point having variable transmitter power levels, transmitter power data is sent from the access points to the mobile units and used by the mobile units in order to provide a corresponding variation in the transmitter power of the mobile unit.

BACKGROUND OF THE INVENTION

This invention relates to wireless local area networks (WLAN) andparticularly, to avoidance of radio interference in such networks. Asdata rates of WLAN systems increase, more access points are required andthe effective range of a higher data rate signals between the accesspoint and mobile units is reduced. In order to provide sufficient highbandwidth connectivity for mobile units, the density of access pointpopulation is increased, particularly, in areas in which a high numberof mobile units are anticipated. The close distribution of access pointsis sometimes referred to as micro-cell distribution. As the distancebetween access points is reduced, RF interference between access pointscan increase. One technique for controlling such interference is reducethe transmit power output of the access points, to thereby reduce theirrange and reduce interference.

Interference can also exist between transmissions of mobile units. It isdifficult, however, to generally reduce the power output of mobileunits, because the mobile unit may sometimes be used in an area wherethere is a high density of access points, and accordingly, only a lowertransmittal power is required, and also the same mobile units may roaminto an area in which access points are spaced at further distances, andthe full power of the mobile unit is required to effectively transmit tothe access points.

It is an object of the present invention to provide a method andapparatus for controlling the transmit power level of a mobile unit inaccordance with the environment encountered by the multiple unit.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a method for use ina wireless local area network, wherein mobile units receive beaconsignals from access points and associate with access points for datacommunication therewith. The method is for controlling transmitter powerlevel of a mobile unit. Data representing transmitter power level for anaccess point is transmitted from the access point to the mobile unit.The mobile unit receives the access point transmitter power level dataand adjusts the power level of the mobile unit transmitter in accordancewith the value of the access point transmitter power level.

The power adjustment of the made mobile unit, may comprise setting themobile unit power level to a power level corresponding to the powerlevel of the access point as represented by the access point power leveldata. Alternatively, the power level of the mobile unit may be set to alevel which is a selected amount greater than the access power leveldata. In one arrangement, the access point power level data istransmitted as part of the beacon signal. The adjustment of the powerlevel of the mobile unit is advantageously performed when the mobileunit associates with an access point.

In accordance with the invention, there is provided a mobile unit foruse in a wireless local area network having access points transmittingat different power levels. The mobile unit includes a receiver forreceiving data signals from the access point and a transmitterresponsive to transmitter power level signals to transmit at a selectedpower level corresponding to the power level signals. The mobile unitfurther includes a processor coupled to receive data signals from thereceiver and programmed to derive power level data from signals receivedfrom the access points and to provide corresponding power level signalsto the transmitter.

In a preferred arrangement, the processor is further programmed toprovide maximum power level signals to the transmitter when it fails toderive power level data from the received AP signals.

In accordance with the invention, there is provided a method forcontrolling interference in a wireless local area network having accesspoints and mobile units. Access points are provided with adjustabletransmitter power level and distributed over an area with varyingspacing between access point. The transmitter power of the access pointsis manually adjusted according to the access point spacing, whereinaccess points with greater spacing have greater transmitter power. Powerlevel data representing transmitter power of the access points istransmitted from each of the points in every 802.11 beacon. Mobile unitsare provided having transmitters responsive to power level signals foradjusting transmitter power of the mobile units. Power level datatransmitted by the access points is received by the mobile units whichprovide power level signals corresponding thereto to the mobile unittransmitters.

In a preferred arrangement, the power level data signals from the APrepresent the same power level as the power level data for the MU, oralternatively, a power level incrementally greater than the power levelrepresented by the power level data. In a system wherein, the accesspoints transmit beacon signals for use by mobile units to associate withthe access points, the power level data can be transmitted within thebeacon signals. Preferably the power level signals are providing to themobile unit transmitter while the mobile unit is within range of theassociated access point. Where the mobile unit does not receive powerlevel data from the associated access point, a maximum power level datasignals can be provided to the mobile unit transmitter.

For a better understanding of the present invention, together with otherand further objects, reference is made to the following description,taken in conjunction with the accompanying drawings, and its scope willbe pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an area map showing the distribution of access points havingfixed and/or variable transmitter power and mobile units forcommunicating therewith.

FIG. 2 is a block diagram of a variable transmitter power access point.

FIG. 3 is a block diagram of a variable transmitter power mobile unit inaccordance with the present invention.

FIG. 4 is a flow diagram illustrating one embodiment of the method ofthe present invention.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a map illustrating exemplarylocations of fixed power access points 10A through 10E, variable poweraccess points 12A through 12G and mobile units 14A through 14E in awireless local area network. In the network illustrated in FIG. 1,access points 10A through 10E may have fixed transmitter power and arearranged at a wider spacing between adjacent access points, which isselected according to the transmitter power, and hence the range of eachaccess point 10. Variable power access points 12A through 12G areprovided in a central area, wherein a greater volume of data traffic maybe anticipated. For example, variable power access points 12A through12G may be provided for servicing mobile units within a centrallaboratory area of an industrial complex, wherein a high volume of datatransmission might be anticipated, while access points 10A through 10Emay be provided in peripheral areas, wherein a smaller number of mobileunits, and hence a lower volume of data traffic may be anticipated.

In the system illustrated in FIG. 1, since variable power access points12 are arranged with a closer spacing between access points, and hence agreater access points population, it can be anticipated that signalsfrom access points, such as access point 12A, may interfere with signalsfrom the adjacent and second adjacent access points, some of which maybe operating in the same frequency band. In order to reduce such radiointerference, or the existence of radio frequency noise in thecommunication channel, the transmitter power of access points 12 isvariable such that the power level of the access points 12 can be set ata value that is reduced according to the reduced spacing of the accesspoints. Fixed transmitter power access points 10, which may also bevariable power access points set at maximum power, transmit a greaterpower level because of the greater distance between access points andthe likelihood that reduced transmitter power will cause coverage gapsin the wireless local area network.

While it is clearly possible to vary the transmitter power of thevariable power access points 12, according to the spacing between accesspoints to provide effective local area network coverage, the fact thatmobile units 14 can roam within any area of the wireless local areanetwork, and hence be at a position which is widely spaced from thenearest access point with which it associates, it is not possible toadjust the transmitter power of the mobile units based their location,since their location is not fixed.

In accordance with the method of the present invention, the variablepower access points 12 transmit power level data to their associatedmobile units, which indicates to the associated or potentiallyassociated mobile units, the power level at which the access point istransmitting. Mobile units 14 receive the power level data from theaccess points and are arranged to adjust the power level of their owntransmitters to correspond to the power level being transmitted by theaccess point with which the mobile unit is or becomes associated. In theevent, the mobile unit, such as mobile unit 14E is associated or becomesassociated with a fixed power access point 10E, of conventional design,it does not receive a power level data from the access point. In thisevent, the transmitter of mobile unit 14E is set to the maximum powerlevel setting.

FIG. 2 is a block diagram of an exemplary embodiment of a variableaccess point 12. Access point 12 includes an asynchronous interfacecontroller 18 which sends and receives signals over cable 16 which isconnected to, for example, a central computer or a wired datacommunications network. A central processing unit 20 in access point 12controls the operation of the access point to transmit and receivesignals and to control the operation of access point 12 in connectionwith association of mobile units therewith. A transmitter/receiver 22 isprovided for sending and receiving wireless data communications signalsusing antenna 24. In connection with the variable power access point 12,the transmitter power level of the access point is controlled, forexample, by providing power level data 26 in an allocated memorylocation of CPU 20. The power level data 26 is transferred to a register28 in the transmitter/receiver 22 whereat it is used to control thepower level of the access point during transmission. When the wirelesslocal area network illustrated, for example, in FIG. 1 is installed andtested, the power level data 26 can be provided at each access point 12in accordance with the spacing of the access points and the signal levelof the access points as observed by testing instruments within the areacovered. The variation of power level 26 may be affected, for example,by a control signal provided by a central processor over the wirednetwork cable 16, or alternately the power level can be set byconnecting a control unit to the access point directly and providing thepower level as a control signal for CPU 20, for example, through aserial port thereof. In connection with the testing of the operation ofthe WLAN, the power level data 26 maybe changed or adjusted, asappropriate, for propagation conditions that are observed. In accordancewith the method of the present invention, the transmitter power level ofmobile units 14 is adjusted in accordance with the power level set bythe power level data 26 of the access point transmitters with which themobile units are associated or about to become associated. For example,if the WLAN of FIG. 1 is a network operating in accordance with IEEEstandard 802.11, the access points 10 and 12 transmit beacon signalswhich are used by mobile units 14 to request association. The mobileunits 14 receive beacon signals from nearby access points and observesignal level thereof. The beacon signals optionally include data signalswhich can be used by the mobile units in connection with the associationprocess. Assuming mobile unit 14C of FIG. 1 receives the strongestbeacon signal from variable power access point 12E, mobile unit 14Cadditionally receives within the beacon signal, data representing thepower level 26 at which variable power access point 12E has been set.CPU 20 adds this power level data to the transmitted beacon signal.Referring to FIG. 3, mobile unit 14 includes a mobile device 30, such asa PDA, a notebook computer or a telephone, a communication controller32, which may be a digital signal processor or a microcomputer, and atransmitter/receiver 34, which transmits and receives radio frequencydata communication signals via antenna 36. Transmitter/receiver 34 isarranged to transmit a transmitter power level which is set forth in aregister 40 within the transmitter receiver.

Mobile unit 14 receives a beacon signal from an access point 12 whichincludes transmitter power data transmitted by the access point 12 aspart of the beacon. The transmitter power level data is extracted fromthe received signal in CPU 32 and set forth in a register 38 in CPU 32.This power level data is used to provide transmitter power level signalsto a reister 40 in transmitter/receiver 34 in the mobile unit 14. In oneexample, the transmitter power level signals may represent the sametransmitter power which is represented by the power level data signalsreceived from the access point. Alternately, since the mobile units 14experience a variety of environmental or multipath conditions, the powerlevel signals provided to the transmitter 34 of the mobile unit 14 mayrepresent a transmitted power level that is greater, for example, 1 or 2incremental values greater, than the transmitter power represented bythe power level data signal received from the access point 12.

In a preferred arrangement, CPU 32 is arranged to set the power levelsignals 40 provided to transmitter 34 at a maximum value when no powerlevel data is received from the access point, for example, when theaccess point is not a variable power access point such, as access point10C of FIG. 1.

FIG. 4 is a flow diagram illustrating the method of controlling power ofthe mobile units 14 in accordance with one embodiment of the presentinvention. As shown in FIG. 4, when the mobile unit makes a decision forroaming, such as upon initial initiation of operation, or when signallevels received from an associated access point vary on account ofmovement of the mobile unit, the mobile unit receives beacon signals andselects an access point with which to attempt association. The accesspoint sends the beacon signal with power data which is thereafterextracted from the beacon data by the mobile unit. The power level ofthe mobile unit is set using the power level data received as part ofthe beacon signals, and provided as a power level signal to thetransmitter 34 of the mobile unit. The mobile unit 14 thereafterrequests association with the access point and the access pointprocesses the association request. Thereafter, during data communicationbetween the mobile unit and the associated access point, the transmittedpower of the mobile unit is set by the power level signal provided totransmitter 34 by CPU 32 and corresponds to the power level data sent bythe access point to the mobile unit. This power level is set to the sameor slightly greater power level than the power level represented by thepower level data previously been set by the access point, as describedabove.

While there have been described what are believed to be the preferredembodiments of the present invention, those skilled in the art willrecognize that other and further modifications and changes may be madethereto without departing from the spirit of the invention and it isintended to claim all such changes and modifications as fall within thetrue scope of the invention.

1. In a wireless local area network wherein mobile units receive beacon signals from access points and associate with access points for data communications therewith, a method for controlling transmitter power level of a mobile unit, comprising: transmitting from an access point to said mobile unit beacon signals that also include data representing transmitter power level for said access point; receiving at said mobile unit said access point transmitter power level data; and adjusting transmitter power level of said mobile unit in accordance with the value of said access point transmitter power level data included in said beacon signals, wherein adjusting power level of said mobile unit comprises setting said mobile unit to a power level a selected amount greater than a power level corresponding to said access point transmitter power level data.
 2. A mobile unit for use in a wireless local area network having access points transmitting at different power levels, comprising: a receiver for receiving data signals including beacon signals from said access points; a transmitter responsive to access point power level signals to transmit at a selected transmitter power level corresponding to said access point power level signals; and a processor, coupled to process data signals from said receiver and programmed to derive access point power level data included in said beacon signals received from said access points and to provide corresponding transmitter power level signals to said transmitter, wherein said processor is further programmed to provide maximum power level signals to said transmitter when said processor fails to derive said power level data from said received signals.
 3. A method for controlling interference in a wireless local area network having access points and mobile units, comprising: providing access points including access points with adjustable transmitter power level and distributing said access points over an area with varying spacing between access points; adjusting the transmitter power of said adjustable transmitter power level access points according to said access point spacing, wherein access points with greater spacing have greater transmitter power; providing access points power level data representing transmitter power of said adjustable power level access points, and transmitting said access points power level data from each of said adjustable power level access points; providing mobile units having transmitters responsive to power level signals for adjusting transmitter power of said mobile units; receiving said access points power level data at said mobile units and providing power level signals corresponding thereto to said mobile unit transmitters, wherein said access points transmit beacon signals for use by mobile units to associate with one of said access points, and wherein said access point power level data is transmitted within said beacon signals, wherein said power level data signals represent a power level greater than the power level represented by said power level data.
 4. A method according to claim 3 comprising the further step of providing maximum power level signals to said mobile unit transmitter when said mobile unit does not receive said power level data. 